1. Field of the Invention
The present invention relates generally to optical switches and modulators, and more particularly to an electrode configuration for controlling the electric field applied to an optical waveguide in optical switches and modulators.
2. Related Art
Certain types of waveguide based optical switches, also referred to as optical modulators, are receiving a great deal of attention due to their digital optical output characteristic. These waveguide based optical switches can be controlled through the electro-optic effect. The electro-optic effect is a term used to describe the change in the index of refraction of an optical waveguide that results from an electric field applied through the optical waveguide. Changes in the index of refraction of an optical waveguide affect the propagation of light through the optical waveguide. For example, a change to the index of refraction of an optical waveguide may direct light to propagate in an optical waveguide. Alternatively, a change in the index of refraction of an optical waveguide may direct light away from propagating in an optical waveguide. For a properly designed switch, application of a control voltage, therefore, can cause the optical waveguide to exhibit a digital response function. That is, the optical output power of an optical waveguide yields a step-like curve when it is plotted against increasing control voltage, that is, there are no significant secondary maxima in such a plot.
An advantage of a digital response characteristic is that the switch can be set to direct an optical signal to a given output waveguide without regard to the polarization of the input signal despite the fact that the electro-optic effect is different for the two polarizations. Another advantage of a digital response characteristic is that the output optical power of the optical waveguide is not sensitive to voltage variations or drift in the applied control voltage. Application of an appropriate control voltage allows light to be routed through a desired path of all optical switch. Routing light ill this manner is called switching. When an appropriate control voltage is applied to each of two output waveguides, propagation of light is promoted in one of the output optical waveguides and propagation or light is inhibited in the other output optical waveguide. Optical switches have broad optical bandwidths making them attractive for the switching needs of high data rate transmission systems, including space- and time-division multiplexing. In addition, optical switches may find application in other areas such as optical signal processing. An optical modulator functions much the same as all optical switch because the optical output power in either of the output optical waveguides can be modulated by an appropriate control voltage.
Optical switches have been implemented using x- or y-shaped optical waveguides. A y-shaped optical waveguide consists of three optical waveguides: one input optical waveguide connected to two output optical waveguides. The input waveguide is connected to the output waveguides at an optical waveguide branch start plane. All x-shaped optical waveguide consists of four optical waveguides: two input optical waveguides connected to two output optical waveguides. The input waveguides connect to the output waveguides at an optical waveguide branch start plane. The terminology, "x-shaped" and "y-shaped", is used as a shorthand to describe the multiple waveguides which form an x-shaped or y-shaped optical waveguide respectively. The terms are descriptive of the shapes of the resulting waveguides. Optical switches using all x- or y-shaped optical waveguide exhibit the highly desirable digital behavior described above. Thus, the voltage transfer characteristic of optical switches made using x- or y-shaped optical waveguides looks like a step function.
X- and y-shaped optical waveguides may be manufactured using a birefringent material such as lithium niobate (LiNO.sub.3). A birefringent material causes different polarizations of light to behave differently when they propagate through it. A light beam that is propagating through a waveguide of an optical switch comprises two polarizations (see FIG. 2): a transverse magnetic polarization (TM) 202 and a transverse electric polarization (TE) 204. The two polarizations may be defined anywhere in the plane which forms a right angle to the direction of propagation of light through an optical waveguide. In the field of waveguide devices, axes are conventionally defined with respect to the top surface of the crystal substrate in which the optical waveguide resides. Light with a transverse electric polarization is defined as light having an electric field vector parallel to the top surface of the crystal substrate. In TE polarized light 204, the magnetic field vector, which is perpendicular to the electric field vector, is perpendicular to the top surface of the substrate. Likewise, light with a TM polarization 202 is defined as light having a magnetic field vector parallel to the top surface of the crystal substrate in which the optical waveguides resides. In TM polarized light 202, the electric field vector, which is perpendicular to the magnetic field vector, is perpendicular to the top surface of the crystal substrate.
In order to obtain a digital response function for a polarization independent switch, the power of both the TM and TE polarizations of light 202,204 in a waveguide of an optical switch must be reduced to as near zero as possible. FIG. 2 shows exemplary optical transfer curves for prior art directional coupler switches in z-cut lithium niobate. In order to reduce the power of both the TM and TE polarizations of light 202,204 to as near zero as possible, an increasing control voltage must be applied to the waveguide. Power in the TM polarization of light 202 approaches zero faster than the TE polarization of light 204 (this is apparent from FIG. 2). Therefore, a greater control voltage must be applied to an optical waveguide to reduce the power in the TE polarization of light 204 to zero than need be applied to reduce the TM polarization of light 202 to zero. As greater voltage is applied to the optical waveguide to reduce the TE polarization 204 to zero, the TM polarization 202 rises (see example rise 206 in FIG. 2). The rise 206 in the TM polarization of light 202 results in crosstalk. Crosstalk refers to unwanted power which appears at the output of one optical waveguide when an electric field has been applied to the switch to direct the optical power to the other output optical waveguide. X- and y-shaped optical waveguides are advantageous because they reduce the rise 206 in the power of the TM polarization of light 202 as the control voltage is increased (see FIG. 2B). Optical switches using x- or y-shaped designs, therefore, reduce crosstalk and signal-to-noise problems associated with other kinds of optical switches which do not use x- or y-shaped designs.
Typically, single mode optical waveguides are used in optical switches. A mode refers to the distribution of the light across the width of an optical waveguide. A single mode optical waveguide has a width sufficient to support only a single mode of light propagating thorough it. The mode supported in a single mode optical waveguide appears at the end of the waveguide as a single spot of light. In a multiple mode optical waveguide, the width of the optical waveguide is sufficient to support multiple modes of light. In a multiple mode optical waveguide, multiple spots of light appear at the output of the waveguide. In a single mode x- or y-shaped optical switch, only one mode of light is supported in the optical waveguide of the optical switch. An exception occurs where the x- or y-shaped optical waveguide begins to branch. Two modes are possible at this point because of the larger width of the waveguide where it begins to branch.
Switching occurs in the active region of an x- or y-shaped optical waveguide. The active region of an x- or y-shaped optical waveguide begins in the region where the x- or y-shaped optical waveguide begins to branch (i.e., where the input waveguide(s) connect to the output waveguides) and extends to the end of the electrodes. Applying an electric field across the branches (the separation or split in a x- or y-shaped optical waveguide) of an x- or y-shaped optical waveguide changes the index of refraction in those branches (changing the index of refraction in an x- or y-shaped optical waveguide causes propagation of light in the branches of the x- or y-shaped optical waveguide to be inhibited or promoted). The electric field can be applied such that one branch of an x- or y-shaped optical waveguide promotes propagation of light through it, while the other branch of an x- or y-shaped optical waveguide inhibits propagation of light through it. The branches of an x- or y-shaped optical waveguide are coupled. That is, the power of the light in the inhibiting branch of the waveguide is transferred to the light in the promoting branch such that the promoting branch contains all of the power of the original input light.
An x-switch provides more flexibility in switching than a y-switch because it is a 2.times.2 switch whereas the y-switch is a 1.times.2 or 2.times.1 switch. A 2.times.2 switch has two inputs and two outputs. The flexibility of the 2.times.2 switch derives from its ability to rout either input to either output. Because the x-switch is difficult to design however, the less flexible 2.times.1 or 1.times.2 y-shape design is more common. However, the following discussion about the design of the electrodes apply to an x- switch as well as a y-switch.
Digital optical switches have different electrode and optical waveguide configurations depending on, among other things, the type of crystal substrate from which the electrodes are made. In lithium niobate, the largest electro-optic effect is obtained for electric fields which are parallel to the crystalline z-axis. Thus, for optical waveguides fabricated in x- or y-cut crystalline material, the electrode are placed on the substrate surface adjacent to the waveguides. The waveguides and electrodes are oriented such that the applied electric field is primarily parallel to the surface of the substrate and in the z direction where it passes through the waveguides. For optical waveguides fabricated in z-cut crystalline material, the electrodes are placed in a plane, parallel to and distinct from a plane in which the waveguides are located so that the applied electric field is primarily perpendicular to the surface and in the z direction where it passes through the waveguides.
For digital switches, a first conventional electrode waveguide configuration is a two electrode design. The two electrodes follow the branching of the output waveguides, resulting in a non-constant distance between electrodes as the waveguides branch. The effect of the non-constant distance along the length of the optical switch is that the electric field, which is proportional to applied voltage (V) divided by the distance between the electrodes at a particular point (L), decreases in intensity as L increases. Thus, the waveguides become inefficient very quickly as they separate, that is, as L increases. A further disadvantage of the first conventional electrode configuration is that advantageous voltage-length reduction techniques, such as one described in W. K. Burns, "Shaping the Digital Switch," IEEE Photonics Technology Letters, Vol. 4., pp. 861-863, August 1992, hereby incorporated by reference in its entirety, cannot be applied because such techniques require the application of a constant electric field.
A second conventional electrode waveguide configuration uses a four electrode design. This configuration maintains a constant separation between pairs of electrodes, thereby overcoming the problem of the V/L inefficiency, described above, for the two electrode design. However, this second conventional electrode configuration suffers from other problems. In particular, there is a practical limit as to how close the inner two electrodes can get. The limit is imposed by the resolution of modern photolithographic processes used to create the electrodes. Further, arcing across the point of closest proximity (between the innermost 2 electrodes) becomes a problem when the electric field intensity is sufficiently high. Thus, the electrodes do not extend into the input region of the optical switch. Light propagating in the optical waveguide of an optical switch which employs the second conventional electrode configuration, experiences an abrupt change in the electric field applied to the optical switch at the beginning of the electrodes. An abruptly applied electric field results in increased crosstalk and propagation disturbance in the optical waveguide. Preferably, the electric field should be applied in a smoothly varying way so as to minimize crosstalk and disturbance of mode propagation.
Therefore, what is required is an optical switch having an electrode configuration that overcomes the above-stated problems. The new electrode configuration should apply a constant electric field through a substantial portion of the separation of the output branches (i.e., the branches through which light propagates after switching) of the optical waveguide. Also, the electrodes in the new electrode configuration should control the application of the electric field to provide a smooth application of the electric field without abruptly turning it on or off. The electrode configuration should begin application of the electric field in an input control region of the optical switch.